This invention relates to a woven material of inorganic fiber, and a process for making the same, suitable for reinforcing various kinds of resin and for, specially, use as printed circuit boards.
Use has been made of woven materials of inorganic fiber such as glass, carbon or alumina fiber and those of glass fiber are particularly in wide use for reinforcing resin and recently as printed circuit boards.
However, a printed circuit board prepared from a laminated woven material of inorganic fiber is disadvantageous in that it is subject to interlamina shear and is inferior with regard to surface smoothness.
In the case of glass fibers constituting a woven material, for instance, the fiber comprises several hundred monofilaments each 5 to 9.mu. in diameter and they are slightly twisted to form yarn, which is used as warps and wefts of the woven material. The inorganic fibrous filaments are thus considerably restrained through the twisting and weaving processes.
Consequently, the inorganic fibrous filaments forming warps and wefts restrain each other although they offer an excellent reinforcing effect as a reinforcing material because of the high percentage of inorganic fiber contained therein. In other words, the portions where wefts are passed over and under warps are swollen and, when the woven material is impregnated with resin and formed into a board, surface roughness caused by the woven material will appear, the disadvantage being insufficient surface smoothness.
The above-described surface smoothness is important when woven materials of inorganic fiber are piled up and used as a laminar board and, particularly in the case of a printed circuit board, improved surface smoothness is important for the electronic parts industry because of the improved wiring density and pattern micronization.
Although the surface smoothness in the case of the present standard double-sided board (1.6 mm thick) is about 7 to 9.mu., there is an increasing need to reduce the smoothness to about 3.mu..
Moreover, as several hundred ends of inorganic fibers are twisted and bundled to form a warp or weft, the warps and wefts constituting a woven material are barely impregnated with resin. The fact that the warps and wefts are barely impregnated with resin affects not only the productivity of boards prepared from laminar inorganic fibers directly but also their mechanical, thermal and electric characteristics because of the non-homogeneous distribution of resin.
A problem also exists when woven materials formed of inorganic fiber are stacked in layers, one upon another. In particular, because none of the fibers extend into the area between the layers, adjacent layers are not fully adhesively joined to each other, so that interlaminar shear can occur.
Attempts have been made to improve the construction of yarn forming a woven material, by using a nonwoven fabric or providing the woven material with shearing force using a roller to improve the surface smoothness and impregnability. Various methods also have been attempted to prevent interlamina shear in a woven material of inorganic fiber by needle punching the woven material for raising purposes. In that case of use of a nonwoven fabric, although such methods have been proved effective in the improvement of surface smoothness and prevention of interlamina shear, the adhesive agent used to maintain the form of the non-woven fabric must be compatible with the resin with which the nonwoven fabric is impregnated, because the filaments of the nonwoven fabric have not been inter woven nor restrained with each other. Because the filaments of the nonwoven fabric are not restrained with each other, moreover, less than half (about 30%) of the fibers of the nonwoven fabric are contained in the adhesive and the disadvantage is that the dimensional stability of the nonwoven fabric is reduced because the reinforcing effect is naturally reduced. Improvements in yarn constituting the woven material and the use of a roller for providing shearing force are not satisfactory enough to solve the above-described problems.
Problems are also associated with the use of needle-punching to reduce interlamina shear. Firstly, on the one hand, the needles can completely cut or destroy the yarns of which the material is formed to an extent producing an internal shear the woven material, or on the other hand, because of differences in the density of the needles relative to the woven density of the material, many of the needles may miss the yarns entirely, leaving portions of the woven material where the fibers have not been punched. As a result, it is almost impossible for needle-punching to create a uniform and thick distribution of raised fiber portions on the surface of the woven fabric. Moreover, the cut or destroyed constructive yarn will make the woven material uneven. In the case of the needle punching, the surface smoothness may be reduced because the woven material is composed of portions which have been punched and unpunched.